selftests/bpf: test MSS value returned with bpf_tcp_gen_syncookie

One remaining difference between test_tcp_check_syncookie.sh and
btf_skc_cls_ingress is a small test on the mss value embedded in the
cookie generated with the eBPF helper.

Bring the corresponding test in btf_skc_cls_ingress.

Signed-off-by: Alexis Lothoré (eBPF Foundation) <alexis.lothore@bootlin.com>
Link: https://lore.kernel.org/r/20241020-syncookie-v2-5-2db240225fed@bootlin.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>